CEPHALASPIDyE. 



.33 



disposed coiiceiitncally round a middle point, as in Tt. Crouchii, but this portion of the 

 shield is apparently elevated into a slight convex boss. The surface-striations of IH. 

 rodraius differ considerably from those of Pt. Crouchii. They are very fine, the 

 groovings being only of an inch apart near the centre of the disc, and leaving a 

 rounded dull ridge with crenate margins between them. 



Localities. — This species occurs abundantly at Cradley, near Malvern, Herefordshire ; 

 and in beds of the same horizon at Whitbach and in the neighbourhood. It is found 

 associated with Scaphaspis Lloydii. 



3. Pteraspis Mitchelli, Poivrie. PI. V, figs. 1, 2, 0, 10, II. 



Pteraspis Mitchelli, Powrie. Geologist, vol. vii, pp. 1/0 — 172, 1864. 



— — Lankester, Brit. Assoc. Report, Trans. Sect., p. 58, 186 1 . 



I have not had specimens of this species sufficiently perfect to enable mc to charac-. 

 terise it properly. The specimen figured (PI. V, figs. 6, 10) was briefly noted by ^Ir. 

 Powrie in the 'Geologist,' in 1SG4, and an outline-sketch was given. The lateral cornua 

 are not seen, but the disc, rostrum, and spine are, to a certain extent. The disc appears 

 to be intermediate in form between that of Ft. rostratus and Pt. Crouchii. The specimen 

 and others even less well preserved were obtained by ]\Ir. Powrie from quarries in 

 Forfarshire, where TIeferosfraci had been discovered for the first time in Scotland by 

 the Rev. Hugh Mitchell. 



A few rhomboidal scales also obtained by Mr. Powrie from this locality (PI. V, fig. 1 ) 

 probably belong to this or an allied species. 



§ IX. Sectio B. — OSTEOSTRACI. 



We have now to consider the forms which are represented by the original type 

 Cephalaspis Lyellii of Agassiz, which have bone-lacuna; as structural elements of their 

 shields, a tubercular ornamentation of the surface, and mesially placed orbits. The 

 bodies, with the scales and fins, of some of these fishes have been discovered, and arc of 

 the greatest interest, but are not sufficiently common or well known to allow of their 

 being used in framing generic and specific distinctions. The division into genera of the 

 Osteostraci is based upon the presence or absence of plates in addition to the cephalic 

 shield, and upon the form and relation of this additional piece with regard to the cc>phalic 

 shield. 



The species are most safely distinguished by the character of the superficial tubercular 

 ornamentation ; where that is not possible the general outline of the head nuist be 



